It is known that the absorption wavelength bands peculiar to hydrocarbon-based substances such as methane, ethane, propane, and butane concentrate around the wavelength band of 3 to 5 μm. These hydrocarbon-based substances are gaseous at ordinary temperature and are flammable, and thus they require careful handling. For this reason, in recent years, it has been becoming increasingly common to image, with an infrared camera, contrast differences resulting from such absorption to detect a gas leak or the like that cannot be detected on the basis of visible light. The wavelength band of 3 to 5 μm is one in which the amount of light from the sun is small and the amount of light radiated from objects is small; thus, it is customary to cool sensors so that, even with a small amount of light, imaging can be performed with high sensitivity. Hence the demand for infrared optical systems that can be incorporated in cooled sensors and that provide high transmittance combined with satisfactory optical performance.
Examples of infrared optical systems are seen in Patent Documents 1 and 2 identified below. Patent Document 1 proposes an infrared optical system composed of three lens elements. Patent Document 2 proposes an infrared optical system composed of two lens elements. The infrared optical system disclosed in Patent Document 1 is composed of three germanium lens elements, namely a positive meniscus lens element convex to the object side, a negative lens element, and a positive meniscus lens element convex to the object side, and provides satisfactory optical performance in the wavelength band of 7 to 13 μm. The infrared optical system disclosed in Patent Document 2 is composed of two lens elements, namely a meniscus lens element convex to the image side and a biconvex lens element, both being germanium lens elements and each having an aspherical surface as one surface.